Rated by buyers 
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Seriously, the 3.5 equivalent (Planar Handbook) is the worst piece of garbage ever made. People actually want information and rules on the planes, not adventure sites (which anyone can create themselves). Planar Handbook even referred to this one. So, can you think of any reason why not to buy this instead? Me either.
As far as books go, it has plenty of good spells (although some in Planar Handbook were okay too), it has huge amounts of info on the planes themselves including the various layers of places like Carceri. This also forms a good suppliment with the DMG 3.5 since the former adds details on the traits of the land throughout the plane for exploration purposes, and the latter adds towns and special sites to each plane. By comparison, the only thing Planar Handbook adds (besides the worthlessly contrived touchstones, "dance around the statue 6 times to activate") is planar breaches and some good class abilities for characters.
Rated by buyers -
As a DM in an ongoing campaign, I need all the help I can get. I get 5-6 different supplements a year. Partially to find new challenges and partially to keep abreast of what's happening out there in Gamer Publishing. I really liked this Planer Handbook (3.0) and I never felt the need to upgreade. It has all the info on all the inner and outer planes, atmosphere, who lives there, how to get there etc etc. I'm not sure what the difference is between the 3.0 version and the 3.5 version, but it can't be much. I say stick with this one.
Rated by buyers 
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If you will excuse a brief rant before I begin--why oh why do the game designers nowadays believe I will be mad if they tell me what their own setting is like? Do they think I don't have enough brains to change or tweak something if I don't like it? Everyone seems so bent on not interfering with my GM given right to be the lord of my own games that the companies are terrified to actually give a straight answer as to what their products are actually about.
That said, let me tell you what this book IS (cause the WotC guys NEVER will). This book is the cosmology for the roleplaying setting of Greyhawk. Forgotten Realms has its own cosmology. Dragonlance has its own cosmology. Eberron has its own cosmology. Each setting since third edition came out has its own planes, gods, and everything. If you're looking for one overarching cosmology for every D&D game, there ain't no such animal. Not anymore. Once you get your head around that you will have a lot less headaches and start liking this book a lot more.
Why the switch? There's a couple of good reasons for this. First, toward the end of 2nd edition it was hard to play in a game setting without running into characters who were transplants from some other world, bouncing around either through planeshifting magic, or a spelljamming ship, or a planescaping door from Sigil, or some kind of natural portal. It was far to easy to hop from setting to setting and all the crossovers started to take their toll on the integrity and mood of each campaign world. Second, in the last edition the vast number of gods from every different D&D world (including all the gods from real world mythology) had to be shoehorned into just over a dozen divine planes. Some of the choices made little sense, and all of the planes just got too crowded! Third, there was a lot of fiction produced for the various game lines, and a lot of the authors (Weis & Hickman of Dragonlance and R.A. Salvatore of the Forgotten Realms in particular) loved to add their own little bells and whistles to the planes in the novels. They would talk about planes that didn't exist in the canon setting, or would create unique mechanics for how the planes functioned in their books that didn't mesh very well with what was set up in the books. Now with the cosmologies separated, the characters in a game are likely to be from the same setting, the gods in each setting have plenty of elbow room, sharing their realms with at most half a dozen fellow gods, and the backlog of cosmological oddities due to narrative licence on the part of various authors can be divvied up effectively to each respective setting.
So understanding that, how good is the book? Well the art is fabulous full colour stuff by some of the best artists in the business. The descriptions of each plane are more than lavish, with individual locales set up within each plane that spark the imagination and get one wanting to dive in and play. Acheron is particularly cool: an eternal battleground covering the faces of infinite numbers of scrapmetal cubes drifting in a grey void lit only by the bonfires of various war camps on distant cubes. Often the great masses will slam into one another, spinning again off into space, crushing flat entire armies. Wow!
Unfortunately some of the graphs and explainations of how planes interconnect are pretty confusing, thick stuff. After a while all the coterminous/coexistant/transitive/subjective planes start to run together and the overlapping circle graphs surrounded by large bronze calipers and thick magnifying glasses don't help too much. There are also some features of the cosmology that bend the perceptions a little--like the fact that there are rivers and other geographical features that run from one layer of a plane to another, and even flow onto other planes, often even when the planes are suspended floating in void with no physical connection between them, or sometimes you have mention of infinite planes like Bytopia, that are flat rolling grasslands in the middle and gradually more mountainous until you get toward the edges where the mountains are impossibly tall and craggy (but then it's an infinite plane, right? So how are there "edges" to an infinite plane? Argh. Head...hurts)
The extra races and monster races are awesome. It is particularly great to see GOOD art of githzerai and githyanki (mind you it's not just good, it's sweet!) and the templates are useful. The rules for making your own cosmologies seem a bit weak and appologetic unfortunately--because this is a place where the book could really shine. Likewise the ideas for the Prestige Classes are wonderful, but the actual abilities of each class are fairly lackluster. The special planar spells just struck me as unnecessary and not very thrilling. Where they might have included a section detailing the cosmologies of their other published settings (Forgotten Realms and Dragonlance...Eberron wasn't out yet) which would have quadrupled the value of a book like this, they instead included some "example" alternate cosmologies. Some are very very cool--like the Far Realm and the Great Tree, but most seem like only partly developed afterthoughts. Interestingly the "alternate" cosmologies presented here which are said to be only examples with no tie to the D&D standard cosmology--end up in the Epic Level Handbook with races attributed to them and tied into the Greyhawk history. Wierd.
So final word. I think the book is great and I love having it in my collection. I think it's use is limited greatly by the fact that it only covers the Greyhawk setting, which is unfortunate in the extreme. Moreso I find the whole philosophy of not wanting to challenge the authority of DM's by making any decisions as far as their products go to be infuriating and frustrating. Nonetheless the ideas in this book are very first rate and wonderfully laid out. I just wish the guys who write these books could decide what their settings are like and then have the guts to stick to it!
Rated by buyers 
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I've never understood DnD's reasoning (in any of its incarnations) for building the particular planar structure that it uses for all is worlds. if the Greeks and Babylonian and all those old civilizations made up their own cosmos, why did the creators of DnD think their world's needed to rip off those wonderful inventions instead of making up their own. Now, I realize they have tried from time to time, but no self-respecting DM will use all the junk in this book.
The only useful part of this book is the planar characteristics. They should have spent 90% of the book on that, and 10% on examples. Then they would have gotten a 5 star vote from me.
Rated by buyers -
I picked up this book from Amazon some time ago and find that it is vital to my nwn module creation. Since acquiring this book, I have added into the module the plane of celesta, the plane of hades, the shadow plane and the planes of fire and air. My players LOVE the new areas. They let you use and create new cool critters to throw at them and expand your imagination! It has been SO fun creating these planes because you can use your create a custom area using only your imagination! Spice up your higher lvl chars lives, throw them in the shadow plane, but you will need this book to make sure you get it all right.